1. Field of the Invention
The present invention relates to monitoring roll density, and is more particularly concerned with monitoring the density of a plurality of paper rolls as they are simultaneously but individually, wound on respective winding rolls.
2. Description of the Prior Art
Heretofore, an encoder has been mounted to one of the drums of a two drum winder and a proximity switch is mounted to the core of one of the rotating rolls. In order to determine density, the pulses from the drum encoder are counted for a predetermined number of revolutions of the wound roll, as established by the once per revolution pulse of the proximity switch. The roll diameter can be calculated and the footage of paper can be calculated based on a multiple of paper roll revolutions. Armed with the basis weight of the paper, the diameter change and the added footage of paper, the incremental density added to the roll can be determined. It should be noted that herein the term "drum" is reserved for a center rotating device between an unwinding station and one or more winding stations and which guides a moving web, while the term "roll" is employed for a rotating device which is winding a web and "roll" is used for a rotating device which unwinds a web.
The foregoing method presents two problems when applied to monitoring of more than one roll. First of all, in order to monitor the density of more than one roll of paper, all rolls must together rotate at the same number of revolutions per minute. For a two drum winder, this is not a serious problem since all rolls rotate at approximately the same rate. The density calculated, however, only holds true for the roll directly connected to the proximity switch. A biwind winder, for example, because of its design, could have each roll rotating at substantially different rates. In order for the current systems to be used on a biwind winder, the processor would have to simultaneously service more than one interrupt, or separate processors would be required for each roll to be monitored.
Another problem is that a system of this type calculates density based on a whole number of roll revolutions which means that the roll diameter as paper is added to the roll is not constant, but slightly changes for each sampling period. The time between sampling periods also increases as the paper roll diameter increases for a constant winder speed. Because all rolls may not be rotating at the same speed and because the time between sampling periods increases, these two factors lead to a non-uniform sheet length density determination.